1. Field of the Invention:
The present invention relates to a diversity transmitter/receiver, and in particular, to a diversity transmitter/receiver which uses spread-spectrum coded multiplexing with interleaving error correction for digital radio transmission in which severe multipath fading is frequently a problem.
2. Description of the Related Art:
Diversity receiving is generally required for fading lines in radio communication. Fading can be broadly divided between flat fading and selective fading. In flat fading, multipath propagation does not occur, but rather, the directly received waves themselves undergo fluctuations in amplitude and phase during propagation. In selective fading, on the other hand, multipath propagation occurs and each of the waves arriving by multipath independently undergo fluctuations in amplitude and phase. In this case, the received signal is a synthesized wave made up of the plurality of multipath waves, and therefore, may result in inverse-phase synthesis at particular frequencies due to phase fluctuation conditions. In other words, selective fading (notches) may occur at frequencies within the received spectrum. While the received waveform itself undergoes no distortion in the case of the above-described flat fading, fluctuation in the reception level becomes a problem. In the case of selective fading due to multipath, however, fluctuation in reception level as well as distortion in the received waveform occurs.
Diversity receivers and adaptive equalizing techniques have been used in the prior art to deal with the above-described fading line. While these include various methods, we will here take up as the prior art spread-spectrum transmission, which has been found to be effective against multipath distortion. Spread-spectrum transmission has been used for military applications for achieving robust transmission despite interference waves. However, multipath waves, which have a long delay time, have low correlation with the principal-wave signal. If spread spectrum is applied in such a case, the multipath waves do not correlate with the diffusion coding and are suppressed by the inverse diffusion operation. Essentially, spread spectrum treats multipath waves as interference and can be considered a type of adaptive equalizer. Multipath waves of short delay time, however, have a high correlation with the principal-wave signal and therefore should not be suppressed by inverse diffusion. In this case, the small delay time between the multipath wave and principal wave results in potential fade-out of the received signal due to inverse synthesis. A diversity receiver that uses the non-correlation between a plurality of propagation paths serves as an effective measure against such fade-out.
FIG. 1A illustrates the principles of diversity transmission and reception, while FIGS. 1B, 1C, and 1D show fluctuations in the received field levels of diversity paths 503, 504, and 505, respectively. FIG. 1E shows the fluctuations in received field level for the output (combined diversity) of receiver 502.
Transmitter 501 is assumed to employ one nondirectional antenna for transmission. A model will be considered in which multipath propagation occurs, and radio waves emitted by a nondirectional antenna are propagated along paths including diversity path 504, which is a direct propagation path, and diversity paths 503 and 505, which are reflected waves. In such a case, the paths differ spatially and fading therefore occurs independently for each path to produce the fluctuations in received field level shown in FIGS. 1B, 1C, and 1D. Diversity reception for coping with such a case decreases the probability of fade-out by synthesizing (FIG. 1E) or by selecting portions of each of the diversity branches where fade-out does not occur. This form of diversity takes advantage of the noncorrelation between the propagation paths, and is therefore termed diversity or path diversity. As a means of realizing this path diversity, an adaptive array employing, for example, a plurality of antennas is generally adopted. In other words, a plurality of arriving multipath waves are extracted through directional control of an adaptive array, and diversity synthesis is enabled by maximum ratio synthesis.
FIG. 2 shows the configuration of a spread-spectrum diversity transmitter/receiver in which spread spectrum is combined with diversity, FIG. 2A showing the transmitter and FIG. 2B showing the receiver.
The transmitter is composed of error-correction coder 401, interleave circuit 402, branch circuit 403, N modulators 404.sub.1 -404.sub.N, N spread-spectrum circuits 405.sub.1 -405.sub.N, synthesis circuit 406, transmitter 407, and transmission antenna 408.
The receiver is composed up of N reception antennas 409.sub.1 -409.sub.N, N receivers 410.sub.1 -410.sub.N, N inverse spread-spectrum circuits 411.sub.1 -411.sub.N, diversity synthesis circuit 412, demodulator 413, de-interleave circuit 414, and decoder 415.
The prior art shown in FIG. 2 does not relate specifically to any reference and is simply a combination of coded multiplexing through a spread spectrum, normal diversity synthesis and interleave error correction. For further details regarding each of spread spectrum, diversity, or interleave error correction of the prior art, reference may be made to texts such as "Principles of Secure Communication Systems" by Don J. Torrieri (Artech House).
In the transmitter, error-correction coder 401 and interleave circuit 402 perform coding for-burst error correction. The coded data are split into N branches at branch circuit 403, subjected to primary modulation at modulators 404.sub.1 -404.sub.N, and further diffused at spread-spectrum circuits 405.sub.1 -405.sub.N. Coding/multiplexing is then performed at synthesis circuit 406 using independent diffusion codes for each branch. In other words, N branches of coded multiplexed channels are used in the diversity transmission. Each synthesized wave is transmitted from transmission antenna 408 by way of transmitter 407.
At the receiver, the N branches of signals received at N reception antennas 409.sub.1 -409.sub.N, pass through receivers 410.sub.1 -410.sub.N, and are inverse spreading at inverse spread-spectrum circuits 411.sub.1 -411.sub.N using the diffusion code relevant to each transmission-side branch. These operations result in the extraction and separation of N branches of signals which are then inputted to diversity synthesis circuit 412. It must be noted here that in order to effect diversity, the signal of each branch must have no correlation. More concretely, the N reception antennas must each receive and extract differing multipath waves arriving from different angles. In some cases, noncorrelation between the antennas cannot be guaranteed by installed spacing of the N reception antennas 409.sub.1 -409.sub.N. For example, in the case of an adaptive antenna set up at an integer power of a half-wavelength of the radio frequency, correlation occurs between the received signals of antenna elements. In such a case, it is necessary to provide N subarray processors in the section following receivers 410.sub.1 -410.sub.N, and to extract signals through N buses using N equivalent antenna beam controls. There are no references directly relating to this process, but a combination of the adaptive array and spread-spectrum methods for eliminating interference waves is proposed in "Combination of Adaptive Array Antenna and a Canceller of Interference for Direct-Sequence Spread Spectrum Multiple Access System" by R. Kohno et al. (IEEE Journal of Selected Areas in Communications, Vol. 8 No. 4, May 1990).
Various methods exist for diversity synthesis circuit 412, including a linear synthesis type using an adaptive filter and a type that performs switching diversity. An example of the synthesis type of the prior art is described in Japanese Patent Laid-open No. 167829/1992. In this publication, the relation between discrimination data of a demodulator and branch intervals is used to carry out spread-spectrum diversity synthesis. Demodulator 413 demodulates the primary modulation of the transmitting side, and de-interleave circuit 414 and decoder 415 execute burst error-correcting. Burst error-correction is adopted in the prior art shown in FIG. 2, but because hits tend to occur in a multipath fading propagation path, this is used for dealing with burst errors.
In the above-described prior art, noncorrelative diversity branch received waves are extracted, and therefore, an adaptive array or an adaptive diversity synthesis construction using a plurality of antennas is necessary for effecting diversity synthesis. Accordingly, reception antennas and receivers are required in a number equal to the number of diversity branches, resulting in the drawback of increased scale of the receiving side apparatus. FIG. 1 illustrates one example of multipath routes, but multipath caused by reflectors vary according to the substance of the reflectors themselves, and multipath conditions are not always constant, particularly in land mobile communication. In other words, it cannot be assumed that multipath always exists, and in such cases, there is the problem that decrease of diversity branch degree is accompanied by loss of the path diversity effect.